India
is at a high risk of heat stress-induced health impacts and
economic losses owing to its tropical climate, high population density,
and inadequate adaptive planning. The health impacts of heat stress
across climate zones in India have not been adequately explored. Here,
we examine and report the vulnerability to heat stress in India using
42 years (1979–2020) of meteorological data from ERA-5 and
developed climate-zone-specific percentile-based human comfort class
thresholds. We found that the heat stress is usually 1–4 °C
higher on heatwave (HW) days than on nonheatwave (NHW) days. However,
the stress on NHW days remains considerable and cannot be neglected.
We then showed the association of a newly formulated India heat index
(IHI) with daily all-cause mortality in three cities – Delhi
(semiarid), Varanasi (humid subtropical), and Chennai (tropical wet
and dry), using a semiparametric quasi-Poisson regression model, adjusted
for nonlinear confounding effects of time and PM2.5. The
all-cause mortality risk was enhanced by 8.1% (95% confidence interval,
CI: 6.0–10.3), 5.9% (4.6–7.2), and 8.0% (1.7–14.2)
during “sweltering” days in Varanasi, Delhi, and Chennai,
respectively, relative to “comfortable” days. Across
four age groups, the impact was more severe in Varanasi (ranging from
a 3.2 to 7.5% increase in mortality risk for a unit rise in IHI) than
in Delhi (2.6–4.2% higher risk) and Chennai (0.9–5.7%
higher risk). We observed a 3–6 days lag effect of heat stress
on mortality in these cities. Our results reveal heterogeneity in
heat stress impact across diverse climate zones in India and call
for developing an early warning system keeping in mind these regional
variations.